System and method for quantifying lung compliance in a self-ventilating subject

ABSTRACT

The lung compliance of a subject that is at least partially self-ventilating is determined. The quantification of lung compliance may be an estimation, a measurement, and/or an approximate measurement. The quantification of lung compliance may be enhanced over conventional techniques and/or systems for quantifying lung compliance of self-ventilating subjects in the lung compliance may be quantified relatively accurately without an effort belt or other external sensing device that directly measures diaphragmatic muscle pressure, and without requiring the subject to manually control diaphragmatic muscle pressure. Quantification of lung compliance may be a useful tool in evaluating the health of the subject, including detection of fluid retention associated with developing acute congestive heart failure.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to the quantification of lung compliance in aself-ventilating subject.

2. Description of the Related Art

Systems for quantifying (e.g., measuring, estimating, etc.) lungcompliance in subjects are known. Such systems include ventilatorsystems configured to mechanically ventilate subjects completely. Thesesystems may be implemented, for example, with subjects that areincapable of self-ventilation.

The quantification of lung compliance in a self-ventilating subject isdependent in part on diaphragmatic muscle pressure during respiration.As such, some systems configured to quantify lung compliance in subjectsthat are self-ventilating require the implementation of an effort belt,or some other sensor that provides a direct measurement of diaphragmaticmuscle pressure. Other systems configured to quantify lung compliance inself-ventilating subjects require that the subject be directed and/ortaught to control diaphragmatic muscle pressure manually. However, thistypically requires the subject and/or doctor to perform a specialmaneuver which, if not performed with precision, may negatively impactthe precision and/or accuracy of the estimation of lung compliance.

SUMMARY OF THE INVENTION

One aspect of the invention is related to a system configured toquantify lung compliance of a subject that is at least partiallyself-ventilating. In one embodiment, the system comprises a pressuresupport device, one or more sensors, and one or more processors. Thepressure support device is configured to generate a pressurized flow ofbreathable gas to be delivered to the airway of a subject that is atleast partially self-ventilating. The one or more sensors are configuredto generate one or more output signals conveying information about oneor more parameters of the pressurized flow of breathable gas. The one ormore processors are operatively linked with the pressure support deviceand the one or more sensors, and are configured to execute one or morecomputer program modules. The one or more computer program modulescomprise a control module, a pressure module, and a compliance module.The control module is configured to control the pressure support deviceto adjust pressure of the pressurized flow of breathable gas during aseries of consecutive inhalations of the subject. The pressure module isconfigured to determine the pressure to which the pressurized flow ofbreathable gas should be adjusted by the control module duringinhalations of the subject during the series of consecutive inhalationssuch that for a first inhalation the pressure is adjusted to a firstpressure and for a second inhalation proximate in time to the firstinhalation the pressure is adjusted to a second pressure that isdifferent from the first pressure, wherein the pressure module randomlyor pseudo randomly determines one or more of (i) the position of thefirst inhalation and the second inhalation within the series ofinhalations, (ii) the first pressure, (iii) the second pressure, or (iv)a difference in pressure between the first pressure and the secondpressure. The compliance module is configured to quantify lungcompliance of the subject based on the difference between the firstpressure and the second pressure and the one or more output signalsgenerated by the one or more sensors during the first inhalation and thesecond inhalation.

Another aspect of the invention relates to a method of quantifying lungcompliance of a subject that is at least partially self-ventilating. Inone embodiment, the method comprises delivering a pressurized flow ofbreathable gas to the airway of a subject that is at least partiallyself-ventilating; generating one or more output signals conveyinginformation about one or more parameters of the pressurized flow ofbreathable gas; determining pressures to which the pressurized flow ofbreathable gas should be adjusted during a series of consecutiveinhalations of the subject, including determining a first pressure for afirst inhalation and determining a second pressure for a secondinhalation proximate in time to the first inhalation such that one ormore of (i) the position of the first inhalation and the secondinhalation within the series of inhalations, (ii) the first pressure,(iii) the second pressure, or (iv) a difference in pressure between thefirst pressure and the second pressure are determined randomly or pseudorandomly; adjusting the pressure of the pressurized flow of breathablegas to the determined pressures during the series of consecutiveinhalations; and quantifying lung compliance of the subject based on thedifference between the first pressure and the second pressure and theone or more output signals generated during the first inhalation and thesecond inhalation.

Another aspect of the invention relates to a system configured toquantify lung compliance of a subject that is at least partiallyself-ventilating. In one embodiment, the system comprises means fordelivering a pressurized flow of breathable gas to the airway of asubject that is at least partially self-ventilating; means forgenerating one or more output signals conveying information about one ormore parameters of the pressurized flow of breathable gas; means fordetermining pressures to which the pressurized flow of breathable gasshould be adjusted during a series of consecutive inhalations of thesubject, including means for determining a first pressure for a firstinhalation and determining a second pressure for a second inhalationproximate in time to the first inhalation such that one or more of (i)the position of the first inhalation and the second inhalation withinthe series of inhalations, (ii) the first pressure, (iii) the secondpressure, or (iv) a difference in pressure between the first pressureand the second pressure are determined randomly or pseudo randomly;means for adjusting the pressure of the pressurized flow of breathablegas to the determined pressures during the series of consecutiveinhalations; and means for quantifying lung compliance of the subjectbased on the difference between the first pressure and the secondpressure and the one or more output signals generated during the firstinhalation and the second inhalation.

These and other objects, features, and characteristics of the presentinvention, as well as the methods of operation and functions of therelated elements of structure and the combination of parts and economiesof manufacture, will become more apparent upon consideration of thefollowing description and the appended claims with reference to theaccompanying drawings, all of which form a part of this specification,wherein like reference numerals designate corresponding parts in thevarious figures. It is to be expressly understood that the drawings arefor the purpose of illustration and description only and are not alimitation of the invention. In addition, it should be appreciated thatstructural features shown or described in any one embodiment herein canbe used in other embodiments as well. It is to be expressly understood,however, that the drawings are for the purpose of illustration anddescription only and are not intended as a definition of the limits ofthe invention. As used in the specification and in the claims, thesingular form of “a”, “an”, and “the” include plural referents unlessthe context clearly dictates otherwise.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a system configured to quantify lung compliance of asubject that is at least partially self-ventilating, according to one ormore embodiments of the invention.

FIG. 2 illustrates a plot of pressure of a pressurized flow ofbreathable gas against time, in accordance with one or more embodimentsof the invention.

FIG. 3 is a schematic diagram of a lung-ventilator circuit, according toone or more embodiments of the invention.

FIG. 4 illustrates a plot of volume difference against time duringinhalation, in accordance with one or more embodiments of the invention.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

FIG. 1 illustrates a system 10 configured to quantify lung compliance ofa subject 12 that is at least partially self-ventilating. Thequantification of lung compliance may be an estimation, a measurement,and/or an approximate measurement. The quantification of lung complianceby system 10 may be enhanced over conventional systems for quantifyinglung compliance of self-ventilating subjects in that system 10 mayquantify lung compliance relatively accurately without an effort belt orother external sensing device that directly measures diaphragmaticmuscle pressure. Quantification of lung compliance may be a useful toolin evaluating the health of subject 12, including detection of fluidretention associated with developing acute congestive heart failure. Inone embodiment, system 10 includes one or more of a pressure supportdevice 14, electronic storage 16, a user interface 18, one or moresensors 20, a processor 22, and/or other components.

In one embodiment, pressure support device 14 is configured to generatea pressurized flow of breathable gas for delivery to the airway ofsubject 12. The pressure support device 14 may control one or moreparameters of the pressurized flow of breathable gas (e.g., flow rate,pressure, volume, humidity, temperature, composition, etc.) fortherapeutic purposes, or for other purposes. By way of non-limitingexample, pressure support device 14 may be configured to control thepressure of the pressurized flow of breathable gas to provide pressuresupport to the airway of subject 12. The pressure support device 14 mayinclude a positive pressure support device such as, for example, thedevice described in U.S. Pat. No. 6,105,575, hereby incorporated byreference in its entirety.

The pressure support device 14 may be configured to generate thepressurized flow of breathable gas according to one or more modes. Anon-limiting example of one such mode is Continuous Positive AirwayPressure (CPAP). CPAP has been used for many years and has proven to behelpful in promoting regular breathing. Another mode for generating thepressurized flow of breathable gas is Inspiratory Positive Air Pressure(IPAP). One example of the IPAP mode is bi-level positive air pressuremode (BIPAP®). In bi-level positive air pressure mode, two levels ofpositive air pressure (HI and LO) are supplied to a patient. Other modesof generating the pressurized flow of breathable gas are contemplated.Generally, the timing of the HI and LO levels of pressure are controlledsuch that the HI level of positive air pressure is delivered to subject12 during inhalation and the LO level of pressure is delivered tosubject 12 during exhalation.

The pressurized flow of breathable gas is delivered to the airway ofsubject 12 via a subject interface 24. Subject interface 24 isconfigured to communicate the pressurized flow of breathable gasgenerated by pressure support device 14 to the airway of subject 12. Assuch, subject interface 24 includes a conduit 26 and an interfaceappliance 28. Conduit conveys the pressurized flow of breathable gas tointerface appliance 28, and interface appliance 28 delivers thepressurized flow of breathable gas to the airway of subject 12. Someexamples of interface appliance 28 may include, for example, anendotracheal tube, a nasal cannula, a tracheotomy tube, a nasal mask, anasal/oral mask, a full face mask, a total face mask, or other interfaceappliances that communication a flow of gas with an airway of a subject.The present invention is not limited to these examples, and contemplatesdelivery of the pressurized flow of breathable gas to subject 12 usingany subject interface.

In one embodiment, electronic storage 16 comprises electronic storagemedia that electronically stores information. The electronically storagemedia of electronic storage 16 may include one or both of system storagethat is provided integrally (i.e., substantially non-removable) withsystem 10 and/or removable storage that is removably connectable tosystem 10 via, for example, a port (e.g., a USB port, a firewire port,etc.) or a drive (e.g., a disk drive, etc.). Electronic storage 16 mayinclude one or more of optically readable storage media (e.g., opticaldisks, etc.), magnetically readable storage media (e.g., magnetic tape,magnetic hard drive, floppy drive, etc.), electrical charge-basedstorage media (e.g., EEPROM, RAM, etc.), solid-state storage media(e.g., flash drive, etc.), and/or other electronically readable storagemedia. Electronic storage 16 may store software algorithms, informationdetermined by processor 22, information received via user interface 18,and/or other information that enables system 10 to function properly.Electronic storage 16 may be (in whole or in part) a separate componentwithin system 10, or electronic storage 16 may be provided (in whole orin part) integrally with one or more other components of system 10(e.g., device 14, user interface 18, processor 22, etc.).

User interface 18 is configured to provide an interface between system10 and subject 12 through which subject 12 may provide information toand receive information from system 10. This enables data, results,and/or instructions and any other communicable items, collectivelyreferred to as “information,” to be communicated between the subject 12and one or more of device 14, electronic storage 16, and/or processor22. Examples of interface devices suitable for inclusion in userinterface 18 include a keypad, buttons, switches, a keyboard, knobs,levers, a display screen, a touch screen, speakers, a microphone, anindicator light, an audible alarm, a printer, and/or other interfacedevices. In one embodiment, user interface 18 includes a plurality ofseparate interfaces. In one embodiment, user interface 18 includes atleast one interface that is provided integrally with device 14.

It is to be understood that other communication techniques, eitherhard-wired or wireless, are also contemplated by the present inventionas user interface 18. For example, the present invention contemplatesthat user interface 18 may be integrated with a removable storageinterface provided by electronic storage 16. In this example,information may be loaded into system 10 from removable storage (e.g., asmart card, a flash drive, a removable disk, etc.) that enables theuser(s) to customize the implementation of system 10. Other exemplaryinput devices and techniques adapted for use with system 10 as userinterface 18 include, but are not limited to, an RS-232 port, RF link,an IR link, modem (telephone, cable or other). In short, any techniquefor communicating information with system 10 is contemplated by thepresent invention as user interface 18.

One or more sensors 20 are configured to generate one or more outputsignals conveying information related to one or more parameters of thepressurized flow of breathable gas. The one or more parameters mayinclude, for example, one or more of a flow rate, a volume, a pressure,a composition (e.g., concentration(s) of one or more constituents),humidity, temperature, acceleration, velocity, acoustics, changes in aparameter indicative of respiration, and/or other gas parameters. Thesensors 20 may include one or more sensors that measure such parametersdirectly (e.g., through fluid communication with the pressurized flow ofbreathable gas at pressure support device 14 or in subject interface24). The sensors 20 may include one or more sensors that generate outputsignals related to one or more parameters of the pressurized flow ofbreathable gas indirectly. For example, one or more of sensors 20 maygenerate an output based on an operating parameter of pressure supportdevice 14 (e.g., a motor current, voltage, rotational velocity, and/orother operating parameters), and/or other sensors. Although sensors 20are illustrated at a single location at or adjacent to pressure supportdevice 14, this is not intended to be limiting. The sensors 20 mayinclude sensors disposed in a plurality of locations, such as forexample, within pressure support device 14, within (or in communicationwith) conduit 26, within (or in communication with) interface appliance28, and/or other locations.

Processor 22 is configured to provide information processingcapabilities in system 10. As such, processor 22 may include one or moreof a digital processor, an analog processor, a digital circuit designedto process information, an analog circuit designed to processinformation, a state machine, and/or other mechanisms for electronicallyprocessing information. Although processor 22 is shown in FIG. 1 as asingle entity, this is for illustrative purposes only. In someimplementations, processor 22 may include a plurality of processingunits. These processing units may be physically located within the samedevice (e.g., pressure support device 14), or processor 22 may representprocessing functionality of a plurality of devices operating incoordination.

As is shown in FIG. 1, processor 22 may be configured to execute one ormore computer program modules. The one or more computer program modulesmay include one or more of a breathing parameter module 30, a controlmodule 32, a pressure module 34, a compliance module 36, and/or othermodules. Processor 22 may be configured to execute modules 30, 32, 34,and/or 36 by software; hardware; firmware; some combination of software,hardware, and/or firmware; and/or other mechanisms for configuringprocessing capabilities on processor 22.

It should be appreciated that although modules 30, 32, 34, and 36 areillustrated in FIG. 1 as being co-located within a single processingunit, in implementations in which processor 22 includes multipleprocessing units, one or more of modules 30, 32, 34, and/or 36 may belocated remotely from the other modules. The description of thefunctionality provided by the different modules 30, 32, 34, and/or 36described below is for illustrative purposes, and is not intended to belimiting, as any of modules 30, 32, 34, and/or 36 may provide more orless functionality than is described. For example, one or more ofmodules 30, 32, 34, and/or 36 may be eliminated, and some or all of itsfunctionality may be provided by other ones of modules 30, 32, 34,and/or 36. As another example, processor 22 may be configured to executeone or more additional modules that may perform some or all of thefunctionality attributed below to one of modules 30, 32, 34, and/or 36.

The breathing parameter module 30 is configured to determine one or morebreathing parameters of the subject. The one or more breathingparameters are determined based on the one or more output signalsgenerated by sensors 20. The one or more breathing parameters mayinclude, for example, a tidal volume, a peak flow, a flow rate, apressure, a composition, a timing (e.g., beginning and/or end ofinhalation, beginning and/or end of exhalation, etc.), a duration (e.g.,of inhalation, of exhalation, of a single breathing cycle, etc.), abreath rate, a respiration frequency, and/or other parameters. In oneembodiment, breathing parameter module 30 determines the one or morebreathing parameter on a per inhale and/or exhale basis. By way ofnon-limiting example, breathing parameter module 30 may determine atleast one given breathing parameters for each exhalation in a series ofconsecutive exhalations. The at least one given breathing parameter mayinclude, for instance, a tidal volume, a peak flow, and/or otherbreathing parameters.

The control module 32 is configured to control pressure support device14 to adjust one or more parameters of the pressurized flow ofbreathable gas. For example, control module 32 may control pressuresupport device 14 to adjust a flow rate, pressure, volume, humidity,temperature, composition, and/or other parameters of the pressurizedflow of breathable gas. In one embodiment, control module 32 controlspressure support device 14 to operate in a bi-level positive airpressure mode where pressure is elevated to a HI level during inhalationand reduced to a LO level during exhalation by subject 12. The controlmodule 32 may determine when to trigger changes from HI to LO and viceversa based on detection of breathing transitions by breathing parametermodule 30.

The pressure module 34 is configured to determine the pressure(s) towhich the pressurized flow of breathable gas should be adjusted bycontrol module 32. The pressure of the pressurized flow of breathablegas may be determined by pressure module 34 based on a therapy regime(e.g., for positive airway pressure support), to enable a quantificationof lung compliance, and/or for other purposes. Determining thepressure(s) to which the pressurized flow of breathable gas should beadjusted includes determining the HI and LO pressure levels for abi-level positive air pressure mode.

As is discussed further below, in order to enable a quantification oflung compliance, the pressure of pressurized flow of breathable gasshould be changed between a pair of inhalations that are proximate toeach other in time. As used herein, the pair of inhalations that areproximate in time to each other may include a pair of inhalations thatare directly adjacent (i.e., consecutive without interveninginhalations), or a pair of inhalations that are reasonably close to eachother in time (e.g., within about 2 minutes, within about 1 minute,within about 30 second, within about 15 seconds, etc.). To facilitatesuch a determination, pressure module 34 is configured to determine afirst pressure to which the pressurized flow of breathable gas should beadjusted during a first inhalation, and a second pressure (differentfrom the first pressure) to which the pressurized flow of breathable gasshould be adjusted during a second inhalation that is proximate in timeto the first inhalation. It will be appreciated that in someembodiments, the quantification of lung compliance may be based onmeasurements taken in two breaths that are not proximate in time.Although this may degrade the accuracy and/or precision of thequantification (due to assumptions made about patient physiology and/orrespiratory conditions during the two breaths), such degradation may notbe fatal to the usefulness of the quantification.

In embodiments in which system 10 is operating in a bi-level positiveair pressure mode, control module 32 implements the first pressure asthe HI pressure for the first inhalation, a LO pressure (determined bypressure module 34) for the exhalation(s) between the first inhalationand the second inhalation, and the second pressure as the HI pressurefor the second inhalation. In embodiments in which system 10 isoperating in a CPAP mode, control module 32 transitions between thefirst pressure and the second pressure at the breathing transitionbetween the first inhalation and the exhalation after the firstinhalation, at a point in time between the first inhalation and thesecond inhalation, or at the breathing transition between the exhalationbefore the second inhalation and the second inhalation.

As is discussed further below, in the quantification of lung complianceby system 10, the diaphragmatic muscle pressure of subject 12 is assumedto be the same for the first inhalation and the second inhalation.However, in some instances, if one or more of the transition timing,pressure level(s), and/or pressure difference, for the first pressureand the second pressure are done in a regular, periodic manner, subject12 may begin to subconsciously anticipate this transition. In responseto this anticipation, subject 12 may inadvertently adjust respiratoryeffort (and diaphragmatic muscle pressure) between the first inhalationand the second inhalation. To avoid this effect, pressure module 34 maydetermine the pressure to which the pressurized flow of breathable gasshould be adjusted by control module 32 during a series of consecutivebreaths including the first inhalation and the second inhalation suchthat one or more of (i) the position(s) of the first inhalation and/orthe second inhalation in the series of consecutive breaths, (ii) thefirst pressure, (iii) the second pressure, and/or (iv) the differencebetween the first pressure and the second pressure may be determined ina random or pseudo random manner.

As used herein, the term “pseudo random” refers determinations of one ormore of the parameters set forth above that approximate the propertiesof random number for the purposes of inhibiting anticipation by subject12. This may include schemes in which a pseudo randomly determinedparameter is determined with some periodicity and/or repeatabilityprovided the period at which the parameter is repeated is large enoughto avoid subconscious anticipation by subject 12.

By way of illustration, FIG. 2 illustrates a plot of pressure asdetermined by a pressure module similar to or the same as pressuremodule 34 vs. time over a series of consecutive breaths. During theseries of consecutive breaths, pressure module 34 determines pressure ofthe pressurized flow of breathable gas in accordance with a bi-levelpositive air pressure mode in which pressure is reduced to a LO level 38during exhalations. In the plot shown in FIG. 2, there a number of pairsof directly adjacent pairs of inhalations that could be viewed as thefirst and second inhalations described above. These pairs are labeled inFIG. 2 with reference numeral 40. The position and/or timing of theseinhalation pairs having separate pressure values associated therewith isa random, or pseudo random, distribution designed to inhibit subjectanticipation. Although not depicted in FIG. 2, from the foregoing itshould be appreciated that in addition to determining the positionand/or timing of the first and/or second inhalation within a series ofconsecutive breaths like the one shown in FIG. 2 in a random and/orpseudo random manner, one or more of the pressure(s) during the firstinhalation and/or the second inhalation, and/or the pressure differencebetween the first inhalation and/or the second inhalation may bedetermined in a random and/or pseudo random manner.

Returning to FIG. 1, compliance module 36 is configured to quantify lungcompliance of subject 12 based on the difference between the firstpressure and the second pressure, and the one or more output signalsgenerated by sensors 20 during the first and second inhalations. In oneembodiment, compliance module 36 determines the lung compliance ofsubject 12 by removing diaphragmatic muscle pressure from input-outputequations modeling the respiratory system of subject 12 during the firstinhalation and the second inhalation.

In one embodiment, the quantification of lung compliance by compliancemodule 36 implements a single-compartment lung and ventilator circuitshown in FIG. 3. In FIG. 3 P_(d) represents device pressure (e.g., thepressure of the pressurized flow of breathable gas generated by pressuresupport device 14), R represents the resistance of the respiratorysystem of a subject, P_(alv) represents alveolar pressure, C representscompliance, P_(mus) represents diaphragmatic muscle pressure, and Q_(p)represents the subject flow. In FIG. 4, Δv(t) represents the patientvolume as function of time. C represents compliance of the subject andΔP_(d) represents the measured change in device pressure from one breathto the next. In this model, it is assumed that the resistance of anexhalation port (e.g., exhalation port at interface appliance 28 inFIG. 1) is much greater than a resistance of a hose (e.g., conduit 26 inFIG. 1). Therefore, the pressure within the subject is approximately thesame as the device pressure. Thus, subject pressure is simplyrepresented as the device pressure in the circuit shown in FIG. 3.Further, it is assumed that the patient flow and patient volume can beestimated by using the difference between the measure total flow of thesystem and an estimated (or measured) leak flow.

It will be appreciated that the implementation of a single-compartmentlung model in the description of determining lung compliance is notintended to be limiting. The removal of diaphragmatic muscle pressurefrom equations modeling the function of the respiratory system of asubject is not dependent on this model, but is used herein because it iscomputationally less expensive than more complex models, and simplifiesexplanation.

The transfer function in the s-domain relating patient flow to pressureof the device and the diaphragm of the subject for the circuit in FIG. 3is given by:

$\begin{matrix}{{\frac{Q_{p}(s)}{P(s)} = \frac{Cs}{{RCs} + 1}},} & (1)\end{matrix}$whereP(s)=P _(d)(s)+P _(mus)(s).  (2)

Additionally, the patient volume is given by the equation:

$\begin{matrix}{{V(s)} = {\frac{Q_{p}(s)}{s}.}} & (3)\end{matrix}$

Thus, the transfer function relating the pressure to patient volume isgiven by the equation:

$\begin{matrix}{{\frac{V(s)}{P(s)} = {\left. \frac{C}{{RCs} + 1}\Rightarrow{V(s)} \right. = {{\frac{C}{{RCs} + 1}{P_{d}(s)}} + {\frac{C}{{RCs} + 1}{P_{mus}(s)}}}}};} & (4)\end{matrix}$where the response to P_(mus) is given by the equation:

$\begin{matrix}{{{V_{int}(s)} = {\frac{C}{{Rcs} + 1}{P_{mus}(s)}}};} & (5)\end{matrix}$and where the external response is given by:

$\begin{matrix}{{V_{ext}(s)} = {\frac{C}{{RCs} + 1}{{P_{d}(s)}.}}} & (6)\end{matrix}$

Now, if P_(d)(s) represents the pressure of the pressurized flow ofbreathable gas generated by a pressure support device, and the pressureduring inhalation is varied between a first inhalation and a secondinhalation that are proximate in time (e.g., directly adjacent), thenequation (4) can be written for the first inhalation and the secondinhalation in the following form:

$\begin{matrix}{{{V_{1}(s)} = {{\frac{C}{{RCs} + 1}{P_{d\; 1}(s)}} + {\frac{C}{{RCs} + 1}{P_{{mus}\; 1}(s)}}}};{and}} & (7) \\{{{V_{2}(s)} = {{\frac{C}{{RCs} + 1}{p_{d\; 2}(s)}} + {\frac{C}{{RCs} + 1}{P_{{mus}\; 2}(s)}}}};} & (8)\end{matrix}$where subscripts 1 and 2 correspond to the first inhalation and thesecond inhalation, respectively.

Since P_(mus) is unknown, the part of the total response associated withthe internal response is also unknown. However, if the assumption ismade that P_(mus) is relatively constant between the first inhalationand the second inhalation (since the first and second inhalations areproximate in time), then P_(mus1)(s) can be assumed to be equal toP_(mus2)(s).

Taking the difference between the volume responses in equations (5) and(6), and using the assumption that P_(mus1)(s) is equal to P_(mus2)(s),the unknown internal response can be eliminated to yield the followingcombination of equations (7) and (8):

$\begin{matrix}{{{\Delta\;{V(s)}} = {\frac{C}{{RCs} + 1}\Delta\;{P_{d}(s)}}};} & (9)\end{matrix}$where ΔV(s) is the difference between V₁(s) and V₂(s), and whereΔP_(d)(s) is the difference between P_(d1)(s) and P_(d2)(s).

Since the pressures and volumes, for the two inhalations (and/or theinstantaneous differences therebetween) are known, any one of variousknown numerical estimation techniques can be used to determineresistance R and compliance C. By way of non-limiting example, thetechnique of least squared error could be implemented.

Returning to FIG. 1, compliance module 36 may quantify lung compliancebased on breathing parameter(s) determined by breathing parameter module30 (which are determined from output signals generated by sensors 20),the known value(s) of the first pressure, the second pressure, and/orthe difference between the first pressure and the second pressure in themanner described above. This quantification may then be implemented forone or more of a variety of different uses and/or in a variety ofdifferent contexts. For example, the quantification of lung compliancemay be implemented to preemptively diagnose congestive heart failure, toprescribe treatment, and/or for other purposes.

Although the invention has been described in detail for the purpose ofillustration based on what is currently considered to be the mostpractical and preferred embodiments, it is to be understood that suchdetail is solely for that purpose and that the invention is not limitedto the disclosed embodiments, but, on the contrary, is intended to covermodifications and equivalent arrangements that are within the spirit andscope of the appended claims. For example, it is to be understood thatthe present invention contemplates that, to the extent possible, one ormore features of any embodiment can be combined with one or morefeatures of any other embodiment.

The invention claimed is:
 1. A system configured to quantify lungcompliance of a subject that is at least partially self-ventilating, thesystem comprising: a pressure support device configured to generate apressurized flow of breathable gas to be delivered to the airway of thesubject that is at least partially self-ventilating; one or more sensorsconfigured to generate one or more output signals conveying informationabout one or more parameters of the pressurized flow of breathable gas;and one or more processors operatively linked with the pressure supportdevice and the one or more sensors, the one or more processors beingconfigured to execute one or more computer program modules, the one ormore computer program modules comprising: a control module configured tocontrol the pressure support device to adjust pressure of thepressurized flow of breathable gas during a series of consecutiveinhalations of the subject; a pressure module configured to determinethe pressure to which the pressurized flow of breathable gas should beadjusted by the control module during inhalations of the subject duringthe series of consecutive inhalations such that for a first inhalationthe pressure is adjusted to a first pressure and for a second inhalationproximate in time to the first inhalation the pressure is adjusted to asecond pressure that is different from the first pressure, wherein thepressure module randomly or pseudo randomly determines one or more of(i) the position of the first inhalation and the second inhalationwithin the series of inhalations, (ii) the first pressure, (iii) thesecond pressure, or (iv) a difference in pressure between the firstpressure and the second pressure; a compliance module configured toquantify lung compliance of the subject based on the difference betweenthe first pressure and the second pressure and the one or more outputsignals generated by the one or more sensors during the first inhalationand the second inhalation.
 2. The system of claim 1, wherein the firstpressure and the second pressure are fixed, and wherein the pressuremodule is configured to determine the pressure to which the pressurizedflow of breathable gas should be adjusted by the control module duringinhalations of the subject during the series of consecutive inhalationssuch that for each inhalation in the series of consecutive inhalationsthe pressure is determined by the pressure module to be either the firstpressure or the second pressure.
 3. The system of claim 1, wherein thepressure module is further configured to determine the pressure to whichthe pressurized flow of breathable gas should be adjusted by the controlmodule during exhalations of the subject between the series ofconsecutive inhalations is lower than the first pressure and is lowerthan the second pressure.
 4. The system of claim 1, wherein the one ormodules further comprise a breathing parameter module configured todetermine, based on the one or more output signals of the one or moresensors, one or more breathing parameter of the subject during theseries of consecutive inhalations, and wherein the compliance module isconfigured to quantify lung compliance of the subject based on thedifference between the first pressure and the second pressure and theone or more breathing parameters determined by the breathing parametermodule during the first inhalation and the second inhalation.
 5. Thesystem of claim 4, wherein the one or more breathing parametersdetermined by the breathing parameter module that are implemented by thecompliance module to quantify lung compliance of the subject comprisetidal volume.
 6. A method of quantifying lung compliance of a subjectthat is at least partially self-ventilating, the method comprising:delivering a pressurized flow of breathable gas to the airway of thesubject that is at least partially self-ventilating; generating one ormore output signals conveying information about one or more parametersof the pressurized flow of breathable gas; determining pressures towhich the pressurized flow of breathable gas should be adjusted during aseries of consecutive inhalations of the subject, including determininga first pressure for a first inhalation and determining a secondpressure for a second inhalation proximate in time to the firstinhalation such that one or more of (i) the position of the firstinhalation and the second inhalation within the series of inhalations,(ii) the first pressure, (iii) the second pressure, or (iv) a differencein pressure between the first pressure and the second pressure aredetermined randomly or pseudo randomly; adjusting the pressure of thepressurized flow of breathable gas to the randomly or pseudo randomlydetermined pressures during the series of consecutive inhalations; andquantifying lung compliance of the subject based on the differencebetween the first pressure and the second pressure and the one or moreoutput signals generated during the first inhalation and the secondinhalation.
 7. The method of claim 6, wherein the first pressure and thesecond pressure are fixed, and wherein determining pressures to whichthe pressurized flow of breathable gas should be adjusted during aseries of consecutive inhalations of the subject includes selectingbetween the first pressure and the second pressure for each inhalationin the series of consecutive inhalations.
 8. The method of claim 6,further comprising adjusting the pressure of the pressurized flow ofbreathable gas during exhalations of the subject between the series ofconsecutive inhalations to a pressure that is lower than the firstpressure and is lower than the second pressure.
 9. The method of claim6, further comprising determining, based on the one or more outputsignals, one or more breathing parameter of the subject during theseries of consecutive inhalations, and wherein quantifying lungcompliance of the subject based on the difference between the firstpressure and the second pressure and the one or more output signalsgenerated during the first inhalation and the second inhalationcomprises quantifying lung compliance of the subject based on thedifference between the first pressure and the second pressure and theone or more breathing parameters determined during the first inhalationand the second inhalation.
 10. The method of claim 9, wherein the one ormore breathing parameters comprise tidal volume.
 11. A system configuredto quantify lung compliance of a subject that is at least partiallyself-ventilating, the system comprising: means for delivering apressurized flow of breathable gas to the airway of the subject that isat least partially self-ventilating; means for generating one or moreoutput signals conveying information about one or more parameters of thepressurized flow of breathable gas; means for determining pressures towhich the pressurized flow of breathable gas should be adjusted during aseries of consecutive inhalations of the subject, including means fordetermining a first pressure for a first inhalation and determining asecond pressure for a second inhalation proximate in time to the firstinhalation such that one or more of (i) the position of the firstinhalation and the second inhalation within the series of inhalations,(ii) the first pressure, (iii) the second pressure, or (iv) a differencein pressure between the first pressure and the second pressure aredetermined randomly or pseudo randomly; means for adjusting the pressureof the pressurized flow of breathable gas to the randomly or pseudorandomly determined pressures during the series of consecutiveinhalations; and means for quantifying lung compliance of the subjectbased on the difference between the first pressure and the secondpressure and the one or more output signals generated during the firstinhalation and the second inhalation.
 12. The system of claim 11,wherein the first pressure and the second pressure are fixed, andwherein the means for determining pressures to which the pressurizedflow of breathable gas should be adjusted during a series of consecutiveinhalations of the subject includes means for selecting between thefirst pressure and the second pressure for each inhalation in the seriesof consecutive inhalations.
 13. The system of claim 11, furthercomprising means for adjusting the pressure of the pressurized flow ofbreathable gas during exhalations of the subject between the series ofconsecutive inhalations to a pressure that is lower than the firstpressure and is lower than the second pressure.
 14. The system of claim11, further comprising means for determining, based on the one or moreoutput signals, one or more breathing parameter of the subject duringthe series of consecutive inhalations, and wherein the means forquantifying lung compliance of the subject based on the differencebetween the first pressure and the second pressure and the one or moreoutput signals generated during the first inhalation and the secondinhalation quantifies lung compliance of the subject based on thedifference between the first pressure and the second pressure and theone or more breathing parameters determined during the first inhalationand the second inhalation.
 15. The system of claim 14, wherein the oneor more breathing parameters comprise tidal volume.